1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly, to a liquid crystal display device for extending a diffusing sheet to diffuse light emitted from a light guide plate to an upper surface of an LED substrate. The diffusing sheet is disposed at a lateral side of the light guide plate to cover an overall upper surface of the light guide plate, thereby preventing image degradation by undiffused light.
2. Description of the Related Art
In recent years, with the development of various portable electronic devices such as mobile phones, PDAs, notebook computers, or the like, the requirement for flat panel display devices having characteristics such as light weight, thin profile, and small sizes that can be applied to those portable electronic devices has been gradually increased. For those flat panel display devices, liquid crystal displays (LCD), plasma display panels (PDP), field emission displays (FED), vacuum fluorescent displays (VFD), and the like have been actively developed, but at present liquid crystal display (LCD) devices are primarily used because of their mass production technology, effective driving method, and high-definition and large screen.
The liquid crystal display device, as a transmission-type display device, displays desired images on a screen thereof by controlling the optical transmission using birefringence of liquid crystal molecules. For this purpose, the liquid crystal display device is provided with a backlight unit, which is a light source to transmit through a liquid crystal layer. Typically, the backlight unit can be largely classified into two types.
One is a side-type backlight unit that is provided at a lateral side of the liquid crystal display panel to emit light into a liquid crystal layer, and the other is a direct-type backlight unit that emits light directly into a liquid crystal layer from a lower portion of the liquid crystal display panel.
The side-type backlight unit is provided at a lateral side of the liquid crystal display panel to emit light into a liquid crystal layer through a reflection plate and a light guide plate. Therefore, it is allowed to have a thin thickness, and primarily used in a notebook computer, or the like, for which display devices having a thin thickness are required. The side-type backlight unit has a disadvantage in the application to a large-sized liquid crystal display panel because a lamp for emitting light is located at a lateral side of the liquid crystal display panel, and also has a disadvantage in obtaining high luminance because light is provided through a light guide plate. Consequently, there has been a problem that it is not suitable for large-sized liquid crystal display panels for LCD TVs.
The direct-type backlight unit may be applicable to a large-sized liquid crystal display panel as well as allow the display panel to have a high luminance because light emitted from a lamp is directly provided to a liquid crystal layer and thus in recent years it has been primarily used in the production of liquid crystal display panels for LCD TVs.
On the other hand, in recent years spontaneously emitting light sources such as a light emitting diode instead of a fluorescent lamp have been used for source of the backlight unit. This light source emits RGB monochromatic light and thus it has an advantage of having a wider color reproduction range and reduced driving power consumption when applied to a backlight unit.
FIG. 1 is a cross-sectional view schematically illustrating a structure of a liquid crystal display device in the related art in which a backlight unit having the foregoing light emitting element is provided.
As illustrated in FIG. 1, a liquid crystal display device includes a liquid crystal display panel 10 having a first substrate 1 and a second substrate 3, and a liquid crystal layer (not shown) for displaying an image by applying signals from the outside, an LED unit 52 having light emitting diodes (LEDs) disposed at a lower lateral side of the liquid crystal display panel 10 for emitting light, a light guide plate 20 disposed at a lower portion of the liquid crystal display panel 10 for guiding light emitted from the LED unit 52 to the liquid crystal display panel 10, a diffusing sheet 32 provided between the liquid crystal display panel 10 and the light guide plate 20 for diffusing light guided from the light guide plate 20 to the liquid crystal display panel 10, a prism sheet 34 positioned at an upper portion of the diffusing sheet 32 for condensing the diffused light, a main supporting unit 25 disposed at a low portion of the light guide plate 20 for supporting the light guide plate 20 and the liquid crystal display panel 10, and a lower cover 40 positioned at a lower portion of the main supporting unit 25 for assembling the liquid crystal display panel 10, the light guide plate 20, the diffusing sheet 32, the prism sheet 34, and the main supporting unit 25.
Furthermore, an LED substrate 50 mounted with the LED unit 52 is placed on an upper portion of the light guide plate 20 and the main supporting unit 25, and a reflection plate 28 is formed at the lower cover 40 to reflect light entered into the lower cover 40 to the liquid crystal display panel 10, thereby enhancing the efficiency of light emission.
Though not shown in the drawing, a plurality of pixels are provided in the first substrate 1 of the liquid crystal display panel 10 in which a pixel electrode and a thin-film transistor are formed at each of the pixels, and a common electrode is formed on the second substrate 3, and thus it is possible to display an image by controlling an amount of light transmission traveling the liquid crystal layer by a control of the alignment of liquid crystal molecules when applying a signal through the thin-film transistor from the outside. At this time, polarizing plates (not shown) are attached to the first and second substrates 1, 3, respectively, thereby controlling the polarization direction of light entered into the liquid crystal layer and also light outputted from the liquid crystal layer.
However, a liquid crystal display device having the foregoing configuration has a problem as follows.
Typically, when various elements such as a light guide plate 20, an LED unit 52, a liquid crystal display panel 10, and the like are assembled together by engaging with the main supporting unit 25, each of the elements should be assembled by taking the assembly tolerance into consideration. Accordingly, in case when a liquid crystal display device is assembled by taking the assembly tolerance into consideration, the gap 36 having a width of “x” may be created between the LED substrate 50 and the diffusing sheet 32 as illustrated in FIG. 2, and thus a lower portion of the light guide plate 20 is directly exposed to the diffusing sheet 32 and the prism sheet 34 while not being covered. Light emitted from the LED unit 52 and entered into the light guide plate 20 will be guided by the light guide plate 20, and diffused and condensed through the diffusing sheet 32 and the prism sheet 34, and applied to the liquid crystal display panel 10 in a uniform state. However, as described above, in case of when light outputted from the light guide plate 20 through the gap 36 between the LED substrate 50 and the diffusing sheet 32 is outputted without passing through the diffusing sheet 32 and the prism sheet 34, the undiffused light of the LED unit 52 will be directly supplied to the liquid crystal display panel 10, thereby causing a problem of degrading image quality in the region supplied by the relevant light, because a different type of light will be supplied, which is not diffused and condensed by the diffusing sheet 32 and the prism sheet 34.